1. Field of the Invention
The present invention relates in general to a solid state imaging sensor, a method for manufacturing the solid-state imaging sensor and an imaging device, and relates in particular to a CCD type solid-state imaging sensor (hereafter CCD solid-state image sensor), a manufacturing method for the CCD solid-state image sensor, and an imaging device incorporating the CCD solid-state image sensor.
2. Description of Related Art
The pixel structure for the CCD solid-state image sensor known in the conventional art is for example shown in the cross section of an essential portion in FIG. 9. A line-shaped vertical transfer section 32 is formed at intervals on a silicon (Si) substrate 31. A line-shaped transfer electrode 33 is formed on the silicon substrate 31 directly above the vertical transfer section 32. Further, a discrete sensor 34 for performing photoelectric conversion is formed on the silicon (Si) substrate 31 between the lines of the transfer electrode 33.
A light-impervious film 35 with an aperture is formed directly above the sensor 34 and covers the transfer electrode 33 from above. Light enters the sensor 34 through the aperture 36 of the light-impervious film 35. This light-impervious film 35 functions to block light from entering any portion other than the sensor 34. Further, incident light is focused on the sensor 34 by a so-called on-chip lens (OCL) 37.
In this kind of CCD solid-state image sensor 30, light enters from the edge of the aperture 36 of the light-impervious film 35 so that light may reflect at the boundary between the light-impervious film 35 and the silicon substrate 31 and a portion of the reflected light may enter the vertical transfer section 32 causing a problem known as "smear". An overhang 35a is installed to project over the bottom of said light-impervious film 35 and over the sensor 34 in order to reduce this smear component. In the conventional art, the focal point for the on-chip lens is for instance brought in proximity to the light-receive surface 34a of the sensor 34 by means of the aperture 36 of the light-impervious film 35, or in other words, formed to be at the same height as the overhang 35a.
An imaging device known in the conventional art, is shown for instance in the concept structural view of FIG. 10. An imaging device 50 has an imaging zone 39 with pixels arrayed vertically and horizontally on the silicon substrate 31, and further has a CCD solid-state image sensor 30 with the above mentioned cross sectional structure for these pixels. Said imaging device is further comprised of a camera lens system 40 comprising a diaphragm 42 and an imaging lens 41 installed above the CCD solid-state image sensor 30.
However, in the above-mentioned CCD solid-state image sensor 30, when light passes through the end of the on-chip lens 37 as shown in FIG. 9, a problem occurs in that the light A is reflected from the upper end (shoulder portion) of the light-impervious film 35 and exits on the outer side of the on-chip lens 37 and thus cannot enter the sensor 34 as intended. Further, the light B located more towards the center of the on-chip lens 37 than the light A, is reflected from the side surface of the light-impervious film 35 and also reflected from the overhang 35A for the light-impervious film 35 projecting over the sensor 34 so that light cannot enter the sensor 34. This problem, as is related later is thought due to the installation of the overhang 35A height in proximity to the focusing point of on-chip lens 37. The extent of light reflection at the light-impervious film 35 increases as the light contains more of these oblique light constituents such as the light A and B mentioned above. This increase is particularly drastic when the F number of the camera lens system 40 of the imaging device 50 is set to be a minimum or when the so-called pupil distance s from the diaphragm 42 of the camera lens system 40 to the light-receive surface 34a is short.
Whereupon, moving the on-chip lens closer or farther away from the light-receiving surface 34a of the sensor 34 was attempted as a countermeasure as well as changing the refraction index of the on-chip lens 70 however a portion of the light input is blocked by the light-impervious film 35 due to the structure of the above-mentioned CCD solid-state image sensor 30. Also in this structure, during the light entry process, the light C which is not blocked by the light-impervious film 35 is successfully incident upon the light-receiving surface 34A however a portion is reflected from the surface of the silicon substrate and does not enter the light-receive surface 34a.
Thus in the CCD solid-state image sensor 30 of the conventional art, even if light is concentrated with the on-chip lens 37 towards the sensor 34, this light will be reflected from the surface of the silicon substrate 31 or the light-impervious film 35 and exit on the outer side of the on-chip lens 37, failing to enter the sensor 34. In other words, many constituents of the light do not contribute to device sensitivity thus leading to decline in sensitivity in the CCD solid-state image sensor 31. The light reflected from the surface of the silicon substrate 31 is reported to be 30 percent or more.
Further, when the pupil distance s is short in the camera lens system 40 in the imaging device 50 of the conventional art shown in FIG. 10, light from the on-chip lens 37 of the camera lens system 40 will irradiate (be incident upon) the sensor 34 however, as related before this light contains a particularly large amount of oblique light constituents. Also, in FIG. 10, from among the input light, the concentrated light E.sub.1 (hereafter referred to as the main light beam) which passes through the approximate center of the diaphragm 42, tends to spread out in an increasingly large angle from the center of the imaging zone 39 towards the periphery with respect to the light-receive surface 34a of the sensor 34, and when the distance s as shown in the figure is short, the oblique light constituents contained in the input light clearly increase as the light approaches the periphery of imaging zone 39. Consequently, the focusing point for the light from the on-chip lens deviates a slight amount at a time from the pixels at the approximate center of the aperture 36 of the light-impervious film 35 as the light shifts from the center of the imaging zone 39 towards the periphery. The focusing efficiency on the sensor 34 of the imaging zone 39 in particularly declines along with a drop in sensitivity and the problem of shooting occurs.
In order to resolve these problems, the conventional art attempted shifting the position of the on-chip lens 37 a slight amount at a time from the center of the imaging zone 39 and towards the periphery, thus offsetting or compensating the input light position according to the distance s as shown in FIG. 11, in an enlarged cross section (b) of an essential portion of FIG. 10. However, in this case also, the light E passing the end of the on-chip lens 37 is reflected from the upper edge of the light-impervious film 35 and cannot enter the sensor 34 due to the reason related before in which the focus point of the on-chip lens 37 is located near the same height as the overhang 35a. This method thus failed to adequately improve light focusing efficiency.
However as increasing progress is made in miniaturizing pixel size along with greater compactness of the CCD solid-state image sensor 30, the need for setting the physical thickness of the light-impervious film 35 to a certain extent vertically becomes more essential however, making the light-impervious film 35 thinner is difficult. Further, the smear constituents cannot be increased so that a covering for the light-impervious film 35 above sensor 34 is needed, in other words making an overhang 35 necessary. Accordingly, when the pixel size is further reduced, the position from the edge of the light-impervious film 35 to the light-receive surface of the sensor 34 (to the approximate focus point) becomes correspondingly deeper. As a result, even if light is focused by the on-chip lens, the constituents reflected by the light-impervious film 35 increase just as related above for the light A, B, E so that focusing efficiency onto the sensor 34 declines and sensitivity is reduced. A particularly drastic drop in sensitivity occurs for light with a small pupil distance when the F value is set to a minimum. Accordingly, miniaturizing the size of the pixels presents problems.
Whereupon, the inventors made repeated evaluations on how to resolve the above problems and obtained the following conclusions. When the on-chip lens 37 is moved far from the light-receive surface 34a of the sensor 34 and also when the focus point is set near the height position of the overhang 35a at the aperture 36 of the light-impervious film 35 as shown in FIG. 6A, or restated, when the focal point length is increased, a large deviation in the focus position occurs on the light-receive surface 34a so that of the light entering the sensor 34, the oblique light D.sub.1, D.sub.2, which passed the end of the on-chip lens 37 strikes the overhang 35a of the light-impervious film 35 and is reflected. Accordingly, the left and right oblique light D.sub.1, D.sub.2 do not contribute to the sensitivity of the solid-state imaging sensor.
On the other hand, when the curvature of the on-chip lens is increased and the on-chip lens is brought near the light-receive surface 34a of the sensor 34, and the focus point is set at a position with a height equivalent to the upper end of the light-impervious film 35 (shoulder portion) and above the aperture 36 of the light-impervious film 35 as shown in FIG. 6B, or in other words, when the focus point length is shortened, then the deviation width of the focus point position becomes smaller on the light-receive surface 34a, reflection from the upper edge of the light-impervious film 35 is suppressed and both the left and right oblique light D.sub.1, D.sub.2 can enter at the aperture 38 formed at the side surface on the side of the aperture 36 of the light-impervious film 35 as shown in FIG. 6A. When light is input into the aperture 38, the oblique light D.sub.1, D.sub.2 advances while reflected by the side surface of light-impervious film 35, thus contributing to the sensitivity of the solid-state imaging sensor. This arrangement can reduce the loss of sensitivity from light having many oblique light constituents such as when the F value is set to a minimum or when the light has a short pupil distance due to the camera lens diameter.